The invention relates to a flexible integrated monolithic circuit which comprises flexible circuit elements.
Conventional monolithic integrated circuits comprise widely varying circuit elements such as diodes, transistors, resistors, capacitors, and coils incorporated into or on a common monocrystalline semiconductor wafer. The multiple, complicated construction of the integrated circuit from semiconductor zones, insulating barrier regions, and metallically conducting connections is very thin, often extending no more than a few micrometers in depth, and the electrical charge carriers move in thin layers and channels practically at the chip surface.
Experiments have shown that thin layers of semiconductor material with a layer thickness of less than 50 xcexcm are flexible in themselves, and that integrated circuits with this layer thickness also remain operational after repeated bending stresses. It is only the great thickness, in relation to the depth of the circuit elements, of the semiconductor chip of conventional construction which renders the chips rigid.
If the surface layer with the functional elements and their interconnections is successfully detached from the rigid base, it is possible to manufacture flexible integrated circuits.
Known flexible integrated circuits of this kind have become the target of wide-ranging technological efforts because of a rising demand for small and convenient integrated circuits for electronics on flexible data carriers for the logistical tracking of objects and persons.
The development trend, in particular for chip cards as data storage cards or smart cards, is towards the development of a multifunctional card with data transfer with or without contacts, so that the use of the chip card can be extended to widely varying fields such as payment, health, telecommunication, and security. For this purpose they are fitted with electronic semiconductor circuits of even larger size so as to accommodate more electronic functions on the card. If these semiconductor circuits are implemented in conventional semiconductor technology on a rigid chip, they will tend to crack or break when the user bends the card. There is accordingly a demand for integrated circuits for chip cards with a high degree of integration which are capable of withstanding strong mechanical loads.
A flexible integrated circuit for smart cards is known from U.S. Pat. No. 6,027,958 which integrated circuit comprises a silicon semiconductor material on a silicon dioxide layer, an integrated circuit manufactured with th silicon semiconductor material, an enveloping layer of silicon dioxide or silicon nitride for sealing off the integrated circuit, and a flexible support layer which is connected to the integrated circuit.
A flexible integrated circuit in accordance with U.S. Pat. No. 6,027,958, however, has various disadvantages as regards its manufacture and operation. Thus the manufacturing process comprises a plurality of process steps which substantially increase the manufacturing cost and the reject percentage. Special attention must be given also to the quality of the layers, and in particular to the mutual adhesion thereof and the accompanying delamination effects in the case of flexible integrated circuits. Enveloping layers of SiO2 or Si3N4 are brittle, prone to fractures, tend to show cracks, and have a bad adhesion.
It is accordingly an object of the present invention to provide a flexible integrated circuit which has improved bonding properties and which can be realized in a simple and inexpensive manner in substantially any size.
According to the invention, this object is achieved by means of a flexible integrated monolithic circuit which is substantially formed by flexible circuit elements, connecting elements between the flexible circuit elements, and a flexible coating which comprises at least one layer of a layer material comprising a polymer.
The flexible coating made from a layer material comprising a polymer performs three functions at the same time: it acts as a passivating layer, as a planarizing layer, and as a mechanical support for the integrated circuit elements and their interconnecting elements. The polymer layer serves to passivate and planarize the circuit elements. At the same time, the polymer layer provides the end product with a sufficient mechanical stability so as to serve as the sole support for the integrated monolithic circuit.
It is preferred for the present invention that the polymer is chosen from the group of polyimide, polycarbonate, fluorocarbon, polysulphon, epoxide, phenol, melamine, polyester, and silicon resins or their co-polymers.
It is particularly preferred that the polymer is chosen from the group of polyimide resins. The flexible integrated monolithic circuit according to the invention will show a very low risk of delamination of the layers as a result of this.
In an embodiment of the invention, the layer material comprises a reinforcement material for mechanical reinforcement of the coating.
In another embodiment of the invention, the layer material comprises a thermally conductive filler material. The filler materials for improving the thermal conductivity compensate for the low thermal conductivity of polymeric materials.
The layer material of the flexible integrated monolithic circuit may comprise a pigment for optical screening of the integrated circuit. In a further embodiment of the invention, the layer material comprises an electrically conductive filler material. A coating provided with an electrically conductive filler material renders it possible to manufacture contacts through pressure contacting without the laborious etching of contact holes. In addition, the electrically conductive filler material may at the same time form an electromagnetic screening for the circuit elements.
The coating may also comprise a field plate, again for electromagnetic screening of the integrated circuit.
In a preferred embodiment of the invention, the coating comprises a first layer on a first surface and a second layer on a second surface of the flexible integrated monolithic circuit. The circuit elements are thus embedded in a polymer foil. The two-sided coating reduces the surface stresses and offers a mechanical support for both main surfaces of the flexible integrated monolithic circuit.
In a further preferred embodiment of the invention, the flexible coating of at least one layer of a coating material comprising a polymer is provided with flexible circuit elements on both of its side (or surfaces). This has the following important advantages. Firstly, the packing density may be increased by up to a factor of two. Secondly, it has been observed that there is hardly any tendency of such a device to curl up; it remains perfectly flat. This is probably due to a balancing of stresses on both sides of the flexible coating. This is a very important features as it renders possible all kinds of processing of the device which require that the device to be processed is flat, for example dicing processes or marking processes. In view of this feature, the device preferably has a symmetrical structure with respect to the flexible coating. Finally, an advantage is provided that the flexible coating is transparent, so that flexible circuit elements on both sides of the coating may optically communicate with each other. In that case one of the circuit elements may be a radiation-omitting device comprising, for example, a III-V semiconductor material, whereas the other circuit element may comprise a semiconductor material which is sensitive to the radiation emitted by the one circuit element.
The invention also relates to a method of manufacturing a flexible integrated monolithic circuit whereby integrated monolithic circuit elements and connecting elements are formed in and on a semiconductor substrate, the main surface of the integrated circuit elements facing away from the semiconductor substrate is coated with a polymer resin, and the semiconductor substrate is removed.
The method is based on process steps usual in semiconductor technology and will lead to a flexible integrated monolithic circuit in a small number of process steps.
In a preferred embodiment of a method according to the invention, both sides of the flexible coating are provided with flexible circuit elements in a similar way. This has the important advantages mentioned above. The substrates of the semiconductor bodies comprising the circuit elements may be removed in a similar manner, either in the same or in different process steps.
Particularly preferred is a method whereby the coating comprising a polymer is provided in a spin coating process. A spin coating process has an excellent planarizing effect. The provision of the coating in a spin coating process levels out the surface steps of the integrated circuit elements, and a circuit with a planar surface is obtained.
If the other side of the flexible coating is to be provided with further flexible circuit elements, the coating layer comprising the polymer is preferably provided with a thin (e.g. 0.5 xcexcm thick) layer of silicon dioxide which in its turn is provided with a so-termed primer layer, for example of MOPS (=metacryloxypropyltrimethoxysilane), before a glue is provided thereon to which the further semiconductor body comprising the further (flexible) circuit elements is attached, permanently in this case. Afterwards, the (semiconductor) substrates of the semiconductor bodies on either side of the coating may be removed, either at the same time or separately.